1. Field of the Invention
The present invention provides a method and system for forming a thin film on an inner wall surface of a substrate facing a space formed in the substrate by means of plasma CVD.
2. Related Art Statement
It has been known to form a hard film such as a diamond film or diamond like carbon film on a substrate to impart corrosion resistance to the substrate. Recently, it has been tried to form a hard film on an inner wall surface of a tubular body, whose applications include various products.
Such hard film is formed by so called plasma CVD process. According to the process, for example, a gaseous raw material such as methane gas is introduced with a carrier gas such as hydrogen gas into a furnace for plasma generating reaction. Such raw material gas is then excited by microwave or high frequency wave to generate plasma reaction gas, resulting in chemical reaction on the substrate. The plasma CVD process, however, is useful for forming a specific hard film uniformly on a plate shaped member or the other member having a simple shape. It is, however, difficult to form a hard film uniformly on a substrate having a complicated shape such as an inner wall surface of a tubular member, because the supply of the plasma reaction gas over the whole surface of the substrate tends to be insufficient.
According to Japanese patent publication 62-136,569A, an electrode of graphite is positioned inside of a tubular member, and it was tried to generate plasma reaction gas directly from a gaseous raw material inside of the tubular member. According to the process, a large amount of the plasma reaction gas can be introduced over the inner wall surface of the tubular member. It is thus possible to easily form a relatively thick and hard film uniformly on the inner wall surface of the tubular member.
According to the process described in Japanese patent publication 62-136, 569A, however, it is needed a graphite electrode smaller than the inner diameter of the tubular member. As the inner diameter of the tubular member is smaller, it becomes extremely difficult to produce a graphite electrode usable in the tubular member. Further, for producing hard films on the inner wall surfaces of a plurality of tubular members at the same time, it is needed to prepare a plurality of graphite electrodes each corresponding with each tubular member. The design of a system for plasma CVD becomes thus complicated so that the production of the hard films is made rather complicated.
According to Japanese patent publication 2002-339, 072A, a difference of pressure is generated between the upstream and downstream of the tubular member so that a gas for plasma reaction is flown into the inner space of the tubular member. A DC bias voltage and pulse voltage are applied on the tubular member so that a thin film is successfully formed on the inner wall surface of the tubular member.